Self administered injectable medicaments are often delivered using a variable-dose injection device. Such a device is known from WO 2004/078239 A1. Prior to the injection the user selects the dose that they require according to their prescribed dose and/or their current or expected future physical condition. A typical example would be an insulin delivery device for diabetics where a patient's dose is determined according to their prescribed dose and their expected food intake and activity level. Typically such devices allow the user to select any dose from 1 unit up to the maximum units that the device can deliver, typically 60 units or 80 units for a manual device, such as a pen-type or syringe injection device.
The drug delivery device of WO 2004/078239 A1 comprises a housing for receiving a dose setting mechanism, a cartridge, a dose dial sleeve with an attached dose dial grip, a clicker, a drive sleeve, a clutch for coupling and decoupling the dose dial sleeve and the drive sleeve, a rotatable piston rod and a button which is pressed for injecting a set dose. The full description of the pen-type injection devices disclosed in WO 2004/078239 A1 is incorporated herein by reference.
To dial a dose a user rotates the dose dial grip. With the clicker and clutch means engaged, the drive sleeve, the clicker, the clutch means and the dose dial sleeve rotate with the dose dial grip relative to the housing and relative to the piston rod. Audible and tactile feedback of the dose being dialed is provided by the clicker and the clutch means. Torque is transmitted through saw teeth between the clicker and the clutch means.
A helical groove on the dose dial sleeve and a helical groove in the drive sleeve have the same lead. This allows the dose dial sleeve to extend from the housing and the drive sleeve to climb the piston rod at the same rate. At the limit of travel, a radial stop on the dose dial sleeve engages a stop provided on the housing to prevent further movement. Rotation of the piston rod is prevented due to the opposing directions of overhauled and driven threads on the piston rod.
Should a user inadvertently dial beyond the desired dosage, the pen-type injector allows the dosage to be dialed down without dispense of medicinal product from the cartridge. The dose dial grip is counter rotated. This causes the system to act in reverse. The torque transmitted through the clutch means causes the saw teeth to ride over one another to create the clicks corresponding to dialed dose reduction. Preferably the saw teeth are so disposed that the circumferential extent of each saw tooth corresponds to a unit dose.
When the desired dose has been dialed, the user may then dispense this dose by depressing the button. This displaces the clutch means axially with respect to the dose dial sleeve causing dog teeth of the clutch means to disengage. However the clutch means remains keyed in rotation to the drive sleeve. The dose dial sleeve and associated dose dial grip are now free to rotate. The axial movement deforms a flexible part of the clicker to ensure the saw teeth cannot be overhauled during dispense. This prevents the drive sleeve from rotating with respect to the housing though it is still free to move axially with respect thereto. This deformation is subsequently used to urge the clicker and the clutch back along the drive sleeve to restore the connection between the clutch and the dose dial sleeve when pressure is removed from the button. The longitudinal axial movement of the drive sleeve causes the threaded piston rod to rotate through a threaded opening in a housing insert, thereby to advance the piston in the cartridge.
In other words, the drive sleeve moves longitudinally, i.e. only in the axial direction, during an injection. Because the drive sleeve and the piston rod are engaged via corresponding threads on the outer surface of the piston rod and an internal face of the drive sleeve, the longitudinal movement of the drive sleeve causes the piston rod to rotate. The housing insert with the threaded opening which is engaged with the piston rod via corresponding threads is fixed within the housing, i.e. prevented from rotation. Thus, the rotating piston rod is screwed through the threaded opening in the housing insert, i.e. the piston rod performs a combined rotational and longitudinal movement along a helical path defined by the corresponding threads of the threaded opening and the piston rod.
Once the dialed dose has been dispensed, the dose dial sleeve is prevented from further rotation by contact of a plurality of members extending from the dose dial grip with a corresponding plurality of stops formed in the housing, thus determining a zero dose position.
Such pen type drug delivery devices have been designed and developed to perform regular injections by persons without formal medical training. This is increasingly common among patients having diabetes where self-treatment enables such patients to conduct effective management of their disease. Because the patient, and not the health care worker, may be using such a drug delivery device, one requirement is that the device should be robust in construction. The drug delivery device must also be easy to use both in terms of the drug delivery device manipulation and understanding of the device's operation. This is especially true for diabetics who are required to inject themselves repeatedly with insulin solution and the volume of insulin to be injected may vary from patient to patient and even from injection to injection. For at least this reason, certain diabetics may require drug delivery devices that allow the patient to inject successive measured dosages of the same or perhaps different preset volumes of insulin solution accurately and with minimum dexterity challenges. This presents a further design challenge since, in the case of certain diabetics, users may have impaired vision and/or may be physically infirm with limited dexterity.
In addition to insulin, other medicaments require a minimum dose to be delivered before they are therapeutically effective. A variable-dose device that allows the patient to deliver doses below the therapeutically effective minimum dose creates the possibility that the user may deliver the ineffective doses either by an error of dose calculation or by mistakenly selecting the incorrect dose. Likewise, some medicaments require that a maximum dose is not to be exceeded. This may be for safety reasons such as increased risk or severity of side-effects or excessive or unwanted actions of the medicament. Current variable-dose delivery devices typically have a maximum dose that is limited by the maximum dose that the delivery mechanism can provide, however, this does not necessarily relate to the maximum advised or prescribed dose of the medicament.